This invention relaetes to a heat-sensitive thermal transfer recording sheet having a white background and excellent image storability.
In recent years, against a heat-sensitive recording method using a heat recording apparatus such as a thermal printer, a thermal facsimile or the like, a thermal transfer recording method has been used practically, since said method has advantages in storability, indelibility and solvent resistance of recorded images. The thermal transfer recording method uses a thermal transfer recording sheet comprising a substrate and a heat-meltable ink layer provided thereon. The ink layer of the thermal transfer recording sheet is superimposed on a plain paper, and the ink of the recording sheet is transferred to the plain paper by using a heat supplied from a thermal head of a thermal facsimile or the like, whereby recording is conducted. The thermal transfer recording in a single color (e.g. a black color) is already used practically.
There is also a heat-sensitive thermal transfer recording sheet which can conduct multiple recording and comprises a substrate, a heat-meltable ink layer provided on one side of the substrate and a heat-sensitive color-forming layer provided on the other side of the substrate. In this recording sheet, the thermal transfer layer has a color because the heat-meltable ink used in the layer is a color dye or pigment, and the color is seen through even at the heat-sensitive color-forming layer side. This is undesirable from the appearance standpoint and makes an image developed on the heat-sensitive colorforming layer difficult to distinguish clearly. Hence, various improvements have been made for heat-sensitive thermal transfer recording sheets.
For example, Japanese Patent Unexamined Publication No. 58-78793 discloses a thermal recording sheet for multiple recording comprising a substrate, a heat-sensitive color-forming layer provided on one side of the substrate and a heat-meltable ink layer provided on the other side of the substrate. However, in order for the color of the heat-meltable ink not to be seen through the substrate at the heat-sensitive colorforming layer side, the substrate is subjected to vapor deposition of a metal previously.
Further, Japanese Patent Unexamined Publication No. 55-126482 discloses a heat-sensitive recording sheet for multiple printing having a heat-sensitive colorforming melt transfer layer containing a combination of a leuco dye and a phenolic compound in place of the colored heat-meltable ink layer.
However, the prior art techniques mentioned above have the following various problems.
According to Japanese Patent Unexamined Publication No. 58-78793, since the metal vapor deposition should be applied to the substrate, one more step is required in the production procedure and thus makes the production cost higher. Further, since the colored heat-meltable ink is used, there is another defect in that a component of the thermal transfer layer is transmitted and contaminates an image receiving sheet which is superimposed on the thermal transfer layer during the storage.
According to Japanese Patent Unexamined Publication No. 55-126482, the phenolic compound is used as a substance which reacts with the leuco dye. However, the phenolic compounds disclosed therein are generally highly reactive and readily bring about a color-forming reaction only by mixing with the leuco dye. Therefore, there is a defect in that there takes place background fogging on the coated side during the coating and drying steps.
On the other hand, some of the present inventors have filed a patent application (U.S. patent application Serial No. 820,305) for a heat-sensitive thermal transfer recording sheet overcoming the defects of the prior art mentioned above and with no background fogging. This heat-sensitive thermal transfer recording sheet comprises a substrate, a heat-sensitive color-forming layer formed on a front side of the substrate and a thermal transfer layer formed on a back side of the substrate, both layers containing an electron donative colorless dye and an electron-accepting compound as color forming components. This recording sheet has various excellent properties such as good appearances, good touch, high developed color density, various kinds of hues, but also has defects in that the image storability is poor, e.g. a heat-sensitive color-forming portion and a transferred portion disappear by the action of a plasticizer or additive contained in a plastic such as polyvinyl chloride when contacted with the plastic, or readily disappear by the contact with a chemical contained in foods and cosmetics, or are readily discolored when exposed to sunlight for a short time.